Oil wells and gas wells are typically drilled by a process of rotary drilling. An earth-boring drill bit is mounted on the lower end of a drill string. Weight is applied on the drill bit, and the bit is rotated by rotating the drill string at the surface, by actuation of a downhole motor, or both. The rotating drill bit includes cutting elements that engage the earthen formation to form a borehole. The bit can be guided using an optional directional drilling assembly located downhole in the drill string, to form the borehole along a predetermined path toward a target zone. Hydrocarbon recovery wells can be drilled thousands of feet into the ground.
A bottom hole assembly (BHA) connected to a lower end of a drill string may include a drill bit, a motor to rotate the drill bit, and an axial oscillation tool to provide axial movement of the BHA and/or drill string. An exemplary arrangement uses a positive displacement motor (e.g., a “mud motor” or a “drilling motor”) which is capable of rotating the drill bit even while the drill string does not rotate. For example, in directional drilling operations using a mud motor with a bent housing, the entire drill string including the bent housing, and the drill bit, may be rotated together to drill a straight section. To drill a deviated section, rotation of the drill string may be ceased with the bent housing at a selected rotational orientation, while the drill bit is rotated using just the mud motor. In these systems, high pressure drilling fluid, conventionally referred to as “drilling mud,” is conveyed to the BHA through the drill string. After passing through the BHA, the mud exits through nozzles located in the drill bit and the mud flows back to the surface via an annulus formed between the drill string and a bore hole wall. The mud motor and the axial oscillation tool use the mud flowing through the drill string as their power source.
Drilling without rotation of the drill string may be referred to as sliding, since the non-rotating drill string essentially slides while the borehole is drilled using just the mud motor. The drill string often contacts the bore hole wall while downhole. If an interval of the drill string is moving relative to the bore hole wall, the interval is in a dynamic friction mode and a dynamic friction force is acting upon the interval. If the interval of the drill string is not moving relative to the bore hole wall, the interval is in a static friction mode and a static friction force is acting upon the interval. When the drill string is rotated, the interval is in dynamic friction mode because the drill string is moving relative to the bore hole wall. When the drill string is sliding without rotating, the interval can enter the static friction mode easier than when it is rotating. Because static friction coefficients are typically higher than dynamic friction coefficients, more weight is required to move or unstick the interval of the drill string when the interval is in the static friction mode than when the interval is in the dynamic friction mode. Without a smooth weight transfer to the drill bit, which is associated with the interval being in the dynamic friction mode, the elasticity of the drill string permits a buildup of downward force at a point, or an interval, in the drill string other than the drill bit. When the downward force overcomes the static friction force at the point, or the interval, in the drill string (i.e., unsticks the interval), there is a sudden transfer of downward force transmitted further down the drill string. This results in a lurching or a spike of applied force on the drill bit, which reduces the control the well bore drilling direction.
The bent sub of a mud motor is coupled to the drill string in a position associated with the desired drilling direction before the bent sub is placed downhole. When weight is applied to the drill-bit-and-rock-interface on the bottom of the hole, the tilt of the drill bit encourages the bore hole to be drilled in the direction of the tilt, or toolface direction. The spike of applied force—due to the unsticking of the interval—can also result in a sudden increase in an applied torque on the drill-bit-and-rock-interface, which can cause a reactive twist in the drill string, including the bent sub. Large angular oscillations of the toolface direction are created due to the sudden increase in the applied torque, and control of the drilling direction is lost. The spikes can stall and damage the drilling motor, which results in time spent replacing or repairing the drilling motor. Further, the large angular oscillations can create damaging vibrations in the BHA, which can damage sensors and electronics in down hole tools. This also results in time spent replacing or repairing the downhole tools.
In order to prevent the spike of applied force that often results from the unsticking of the interval—and associated reduced steering ability and possible tool damage—axial loading of the drill string is varied, using the axial oscillation tool, in a cyclical manner. This cyclical axial loading causes continuous longitudinal movement or axial vibration of at least a portion of the drill string and thereby maintains at least a portion of the drill string, or the interval, in the dynamic friction mode.
Often, more than one axial oscillation tool is located in the drill string. Each axial oscillation tool may be positioned along the drill string as the drill string is extended into the bore hole. This allows for each axial oscillation tool to create oscillatory axial drill string vibrations within at least a portion of the drill string. As each axial oscillation tools extends downhole, it passes through multiple areas of the bore hole, with some areas prone to cause sticking that may require larger mud pressure differentials to be created by the axial oscillation tool. As the bore hole lengthens, each axial oscillation moves relative to the bore hole through the multiple areas of the bore hole, with some areas not prone to cause sticking. Additionally, drilling conditions vary such as, for example, the tortuosity of the bore hole changes or the mud is replaced with a mud that has a higher friction coefficient. Without being able to modify operating parameters of each axial oscillation tool while it is downhole, the operating parameters for each axial oscillation tool are set (at the surface) to create large mud pressure differentials so that oscillatory axial drill string vibrations are created in the areas prone to cause sticking. However, this can result in each axial oscillation tool creating large mud pressure differentials in the areas that are not prone to sticking.